The wye-delta connection affords the advantage of the wye-wye connection without the resulting disadvantage of unbalanced voltages and third harmonics in the line-to-neutral voltages when operating without the neutral wire. The wye-delta arrangement is shown in the image below. In high-voltage transmission systems, the high side of a transformer bank or of a 3-phase transformer is generally connected in wye, whereas the low side is connected in delta. The delta connection assures balanced line-to-neutral voltages on the wye side whether or not there is a neutral conductor on the wye side, and it provides a path for the third harmonic components in the exciting current independent of the neutral conductor.

The wye-delta or delta-wye transformation is not confined to applications in which the high-voltage side is connected in wye, but is also coming into general use in the 208/120-v system on the low side. In such systems, the low side is connected wye with the neutral point grounded. Single-phase loads are connected line to neutral for 120-v operation, whereas 3-phase equipment, such as motors, are connected line to line for operation at 208 v.

This vector shows the phasor diagram for a wye-delta transformation. From this diagram it is evident that there is a large phase angle between the line-to-line voltages on the wye side and the corresponding line-to-line voltages on the delta side. This angle is 30° with the phases as designated in Fig. 6-26. Angles of 90° and 150° are possible, depending on how the phases on the two sides are designated.

Great Question... on a wye primary (lineside) and a delta secondary (loadside), When energizing, de-energizing, and testing voltage the primary float must be tied down (grounded). After the bank is placed in opperation the float is opened up (un-grounded).

The reason we do this is because with the float grounded and we lose one of the transformers the two remaining transformers will supply the missing phase.

Now, you may think to yourself, "so what, that seams like a good thing, the customers will never know the difference. They will still have full power on all three phases".

The problem is that only two transformers are supplying the KVA. The bank is now running on only 2/3 of its rated KVA. and No-one will ever know it. You also risk overloading the customers Mains and damaging equipment on the customer side. or burning up one of the remaining transformers.

On large three phase Wye Delta ratio banks It is very important to tie down the float before doing any kind of switching, expecially if your closing one switch at a time. We've had 500KVA ratio banks completly destroyed and entire neighborhoods file claims for damaged equipment. ( Dangerous over voltages can develope) sometimes up to twice the normal phase to ground voltage. At the very least your gonna blow the arrestors of the tanks. Before doing any kind of switching on a wye delta... Make damn sure the primary float is grounded... We install a switch on the float for just this reason. Durring normal opperation you'll notice the grounding switch is open.

With a wire down call, a good rule of thumb is if the trouble is on the line side...Open the line side first. If the trouble is on the load side open the load side first.

On a delta wye bank the float is grounded.(Delta Primary wye secondary) Always (unless you are backfeeding it).

welcome to the site c43 . You are 100% correct. the first post was not my words. I copied and pasted it from a file I saved years ago on my harddrive. Unfortunatly I didn't save the link or the authors name. I found the info very informative and usefull and wanted to share it with the members here. Please accept my deepest and sincere aplologies for not giving credit to the original author. If anyone knows the name of the author please post it so I can edit my post and give credit to the author. The purpose of this site is to share knowledge and help teach the younger guys. Some Lineman in this country have never even seen a wyedelta bank much less built one. Any knowledge we can share here can only help the trade. I will certainly try and include the link and source of any information I post. Thanks again for joining c43 ! I'm looking forward to reading your posts in the future.

topgroove wrote:welcome to the site c43 . You are 100% correct. the first post was not my words. I copied and pasted it from a file I saved years ago on my harddrive. Unfortunatly I didn't save the link or the authors name. I found the info very informative and usefull and wanted to share it with the members here. Please accept my deepest and sincere aplologies for not giving credit to the original author. If anyone knows the name of the author please post it so I can edit my post and give credit to the author. The purpose of this site is to share knowledge and help teach the younger guys. Some Lineman in this country have never even seen a wyedelta bank much less built one. Any knowledge we can share here can only help the trade. I will certainly try and include the link and source of any information I post. Thanks again for joining c43 ! I'm looking forward to reading your posts in the future.